The Iroquois have absorbed many other peoples into their tribes as a result of warfare, adoption of captives, and by offering shelter to displaced peoples. Culturally all are considered members of the clans and tribes into which they are adopted by families.

The historic St. Lawrence Iroquoians, Wyandot (Huron), Erie, and Susquehannock, all independent peoples, also spoke Iroquoian languages. In the larger sense of linguistic families, they are often considered Iroquoian peoples because of their similar languages and cultures, all culturally and linguistically descended from the Proto-Iroquoian people and language; however, they were traditionally enemies of the nations in the Iroquois League. In addition, Cherokee is an Iroquoian language. The Cherokee people are believed to have migrated south from the Great Lakes area in ancient times, settling in the backcountry of the Southeast United States, including what is now Tennessee.

In 2010, more than 45,000 enrolled Six Nations people lived in Canada, and about 80,000 in the United States.

The most common name for the confederacy, Iroquois, is of somewhat obscure origin. The first time it appears in writing is in the account of Samuel de Champlain of his journey to Tadoussac in 1603, where it occurs as "Irocois". Other spellings appearing in the earliest sources include "Erocoise", "Hiroquois", "Hyroquoise", "Irecoies", "Iriquois", "Iroquaes", "Irroquois", and "Yroquois", as the French transliterated the term into their own phonetic system. In the French spoken at the time, this would have been pronounced as or . Over the years, several competing theories have been proposed for this name's ultimate origin— the earliest such proposal is by the Jesuit priest Pierre François Xavier de Charlevoix, who wrote in 1744:

The name Iroquois is purely French, and is formed from the term Hiro or Hero, which means I have said – with which these Indians close all their addresses, as the Latins did of old with their dixi – and of Koué, which is a cry sometimes of sadness, when it is prolonged, and sometimes of joy, when it is pronounced shorter.